For various reasons, including quality issues, the representation of video images in digital form for, e.g., television and other video purposes, is gaining in popularity.
In order to reduce the amount of data required to represent one or more video images in digital form, the use of coding methods which provide for data compression are becoming common place.
Many video coding standards and systems use Discrete Cosine Transform (DCT) coding methods. The MPEG-2 standard that is described in the International Standards Organization--Moving Picture Experts Group, Drafts of Recommendation H.262, ISO/IEC 13818-1 and 13818-2 titled "Information Technology--Generic Coding Of Moving Pictures and Associated Audio" (hereinafter "the November 1993 ISO-MPEG Committee draft"), which is hereby expressly incorporated by reference, is one such standard. References in this application to blocks, macroblocks, P-frames, B-frames and I-frames, unless indicated otherwise, are to be interpreted in a manner that is consistent with the use of the same terms in the MPEG-2 standard referred to above.
While DCT coding and other digital coding methods offer the ability to greatly compress video data, the video images which are generated after using such coding techniques often suffer from some visual artifacts which are the result of the coding process.
One of the most annoying artifacts is referred to as a blocking or blocky artifact. This undesirable artifact derives its name from the fact that it results from the processing of portions of video images in blocks, e.g., 8.times.8 blocks of pixels.
The MPEG standard provides for coding of video images using, among other things, DCT coding. With regard to MPEG coding, fades in video often lead to blocky artifacts which result in a blocky picture because MPEG prediction does not represent fades well. Each frame within the fade portion can not be well predicted from the frame before, so it is as if every frame or image in a scene changes for as long as the fading lasts. Unlike scene changes in a fade, a viewer's eyes can follow the fading images and blocks are noticeable.
Because of data constraints associated with television broadcasts, it is expected that MPEG video used for television will generally not be allocated enough bits to code many sequential frames independently requiring data compression that will result in a noticeable degree of blockiness in the case of fades.
In addition to generating blocky images when video fades are present, MPEG coding also results in blocky images when the frames or images include non-translational motion or large amounts of motion beyond the searched motion vector region. This is because MPEG motion compensated prediction is a poor model for such types of motion.
While various methods of filtering of coded video images have been suggested for reducing the effects of blockiness, most methods have the unfortunate effect of reducing the quality of video images that are not blocky when the blockiness reduction methods are applied to such images.
Accordingly, there is a need for methods of identifying video images which would benefit from the application of processing to decrease blockiness and for improved methods of reducing the effects of blocking artifacts, e.g., of reducing the blockiness of video images without negatively affecting the quality of non-blocky video images.